One objective of the proposed projects is to study the effects of noise on the inner ear epithelia. Changes in the magnitude and pattern of hearing loss and cochlear damage will be determined in experimental animals as the parameters of exposure (frequency, intensity duration and duty cycle) are systematically changed. Attempts will be made to elucidate the mechanisms of noise damage. Normal ears will be exposed to conditions with known mechanisms of action (e.g. changes in ionic environment) and the cellular alterations found in these ears compared to those in noise-exposed specimens. Also various electronmicroscopic tracers will be perfused through the different cochlear scalae in an attempt to determine whether or not permeability of the cochlear duct changes after a damaging noise exposure. Another objective is to define the anatomical correlates of ischemia and other forms of hypoxia and recovery therefrom in the inner ear structures. The morphological changes which signify irreversible damage after repeated episodes of complete or partial anoxia will be identified. Histological evaluation of the specimens will include detailed study of the plastic-embedded flat preparations by phase contrast microscopy so that counts of missing and damaged sensory cells can be made. Specific regions in the organ of Corti will then be sectioned for light and/or transmission electronmicroscopic study so that morphometric analysis of the hair cells, nerve fibers etc. in both normal and damaged cochleae can be made. Some of the experimental animals will be behaviorally trained by food-reward operant-conditioning techniques so that their pure-tone thresholds and several suprathreshold measures of hearing ability can be determined before, during and after their experimental treatment.